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General Chemistry Principles, Patterns, and Applications, 2011

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correspond directly to the lattice energy of the crystalline solid. However, the large negative value indicates that<br />

bringing positive <strong>and</strong> negative ions together is energetically very favorable, whether an ion pair or a crystalline lattice<br />

is formed.<br />

We summarize the important points about ionic bonding:<br />

<br />

At r0, the ions are more stable (have a lower potential energy) than they are at an infinite<br />

internuclear distance. When oppositely charged ions are brought together from r = ∞<br />

to r = r0, the energy of the system is lowered (energy is released).<br />

<br />

Because of the low potential energy at r0, energy must be added to the system to separate<br />

the ions. The amount of energy needed is the bond energy.<br />

<br />

The energy of the system reaches a minimum at a particular internuclear distance (the<br />

bond distance).<br />

E X A M P L E 1<br />

Calculate the amount of energy released when 1 mol of gaseous Li + F − ion pairs is formed from the<br />

separated ions. The observed internuclear distance in the gas phase is 156 pm.<br />

Given: cation <strong>and</strong> anion, amount, <strong>and</strong> internuclear distance<br />

Asked for: energy released from formation of gaseous ion pairs<br />

Strategy:<br />

Substitute the appropriate values into Equation 8.1 to obtain the energy released in the formation of a<br />

single ion pair <strong>and</strong> then multiply this value by Avogadro’s number to obtain the energy released per mole.<br />

Solution:<br />

Inserting the values for Li + F − into Equation 8.1 (where Q 1 = +1, Q 2 = −1, <strong>and</strong> r = 156 pm), we find that the<br />

energy associated with the formation of a single pair of Li + F − ions is<br />

E = kQ1Q2r0 = (2.31´10 - 28 J × m) / (+1)(-1)156 pm ´10 -12 m / pm<br />

= -1.48 ´10 -18 J / ion pair<br />

Then the energy released per mole of Li + F − ion pairs is<br />

E = (-1.48 ´10 -18 J / ion pair)(6.022 ´1023 ion pair / mol) = -891 kJ / mol<br />

Because Li + <strong>and</strong> F − are smaller than Na + <strong>and</strong> Cl − (see Figure 7.9 "Ionic Radii (in Picometers) of the Most<br />

Common Oxidation States of the "), the internuclear distance in LiF is shorter than in NaCl. Consequently,<br />

in accordance with Equation 8.1, much more energy is released when 1 mol of gaseous Li + F − ion pairs is<br />

formed (−891 kJ/mol) than when 1 mol of gaseous Na + Cl − ion pairs is formed (−589 kJ/mol).<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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